Laser scanning systems, such as those used for marking, laser imaging, laser drilling, semiconductor processing, and other material processing, and other similar application of XY scanning, include two mirrors that position a laser beam relative to a target. The laser beam is first directed over a predetermined horizontal range by an X-mirror. The Y-mirror then intercepts the beam and re-directs it over a predetermined vertical range. The Y-mirror must be long enough to intercept the beam over the entire horizontal range associated with the X-mirror. Accordingly, the Y-mirror is generally larger, and thus, heavier and has greater inertia, than the X-mirror.
The two mirrors must move with equal speed for precise re-direction of the laser beam. Accordingly, the maximum speed with which the mirrors can be driven is determined by the maximum rate at which the larger, heavier mirror smoothly accelerates. The rate of acceleration of a given mirror is essentially limited by the resonant frequency of the mirror, since driving the mirror any harder results in imprecise movement. Generally, the larger Y-mirror has a lower resonant frequency, and thus, a slower rate of acceleration.
To maximize the rate of acceleration, the inertia of the mirror should be as low as possible and the mirror should also be as stiff as possible, so that the mirror responds rapidly and precisely to a drive signal. Accordingly, there is a trade-off between minimizing the weight, and thus, the inertia, of the mirror, and maximizing the stiffness of the mirror.
In prior systems, the weight and inertia of the Y-mirror is minimized by undercutting the comers of the mirror. The cuts are typically symmetric about an axis of symmetry, which is perpendicular to the axis of rotation. The length of the mirror along its axis of rotation is fixed by the rotational range of the X-mirror and the beam aperture, and thus, the undercuts must leave intact an elongated center span of the mirror. The stiffness of the mirror is also essentially provided by the center span. Accordingly, the sizes of the corner cuts are determined as a trade-off between minimizing the weight of the mirror and leaving enough material around the center span to provide sufficient stiffness.
The speed with which the mirrors can be driven determines how quickly the system can, for example, produce a desired mark or image, or the number of holes that can be drilled per second, and so forth. There is thus a need for a laser scanning mirror that can be driven rapidly, without adversely affecting the precision with which it is positioned.